MACS Monthly Newsletter - 2021 - MAR3

(lower voltage / lower average voltage courtesy PWM
controls). What happens when the evaporator is severely
restricted as can happen with leaves and greasy dirt (i.e. a
leaky evaporator)? The air output is similarly low, but the
current draw is higher because all that air that the blower
started moving now has a very restricted path to move
through. Current is higher than expected. When it comes
to fluid pumps (coolant and fuel), the same applies. An
unwanted voltage drop will cause lower than expected
current draw as will low coolant flow related cavitation
(low coolant level / air in system) for your cooling system
electric pump and low fuel level / clogged pump strainer)
for the electric fuel pump. Current (amps) level too high
for fluid pumps? A restriction in the cooling / heater system upstream of the auxiliary coolant pump can cause
that. For the fuel pump, the higher than normal current
draw may only be due to a restricted inline filter. Today,
most fuel pumps have these filters built into their modular
assemblies. Another cause of higher than normal current
draw with electric motors is their internal mechanical and

Dave Hobbs

diagnostics, this indisputable law runs into trouble when
the resistance changes. With light bulbs, the resistance
changes as the bulb's element heats up the second it turns
on. Electrical engineers refer to a light bulb's filament as
a non-ohmic element, meaning it changes. The filament
in an incandescent bulb does not have a constant resistance. If you take a bulb and increase the voltage across
it, the current increases too. An increase in current means

Figure 3: You might presume the blower should draw 24
amps (12-Volts / 0.5 Ohms = 24 Amps), but as you can
see, when the fan is powered up and running with 11.45
volts (top meter) it draws over 12 amps (bottom meter).
The actual running resistance can be calculated at just
below 1 ohm. (11.45 Volts / 12.8 amps = 0.9 ohms). This
is why you'll rarely see a resistance spec for an electric
motor.

Figure 2: Ohm's Law Applied on 2-Wire Brushed DC Motor.
If you look at the resistance between the power lead terminal
and ground on this older 12-volt blower motor, it shows 0.5 ohms
at rest. Move the squirrel cage a little and the resistance will be
thrown off and then settle back down as the fan stops.
electrical condition. Brushes that are comprised of carbon
have built in resistance. Wear the brush shorter, that resistance goes down. Bushings can also wear out causing binding of the motor (and more current draw). The fan
blades on a cooling fan can be the wrong ones causing an
increased load on the motor.
7. MOTOR RESISTANCE & OHMS LAW: If you're measuring the resistance of a normal electrical coil (i.e. relay
winding) or simple resistor, the current draw across that
electrical load WILL ALWAYS be proven with Ohm's law.
These devices are sometimes referred to as " ohmic " -
meaning Ohm's law always applies. Example: 12 volts applied to a 20-ohm relay winding will result in 0.6 amps of
current draw. If that relay winding goes down to 2 ohms,
the current draw is now 6 amps (amps = volts divided
by ohms). As technicians in the real world of repair shop

March 2021

the bulb gets hot-hot enough to glow. As the temperature increases, however, the resistance of the bulb's filament also increases. Dynamic changes in electric motors also occur as they spin. As the brushes turn from
one commutator bar to the next, the motor's resistance
(Figure 2) changes when measuring between the power
and ground connections at the motor. So, what does this
mean to you as a technician? It means you can measure
the resistance between the input terminal to most motors
and ground in your diagnostic process. What should you
see? You should NOT see a low ohms reading such as
0.1 ohms (unless the motor is a VERY high current device
such as a starter motor drawing 120 amps) nor should you
see OL (Out of Limit / infinity) which would mean you're

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MACS Monthly Newsletter - 2021

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